The element hydrogen (H) is very efficient moderator of energetic neutrons because of the relatively small difference in their masses. Thermal neutron porosity tools or subsections comprising a neutron source and at least one axially spaced thermal neutron detector is, therefore, very responsive to hydrogen content or “hydrogen index” of the environs in which it is disposed. In a large majority of earth formations, H is within fluid which is in the pore space of the formation. The response of a neutron porosity tool to hydrogen index can, therefore, be used to obtain an indication of pore space and therefore an indication of formation porosity.
Thermal neutron porosity tool response is also affected by borehole conditions such as borehole diameter and the radial position of the tool within the borehole, which is commonly referred to as “standoff”. Tool response is further affected by elements with high thermal neutron cross sections. Examples of such elements are boron which is found in shale, and chlorine which is found in saline formation waters. Corrections applied to thermal neutron porosity measurements for effects such as these are commonly known as “environmental corrections”. Finally, tool response is affected by neutron source strength, thermal neutron detector efficiency, source-detector geometry including shielding and axial spacing, and systematic factors in the electronics associated with the detector. The combination of responses of two thermal neutron detectors at different axial spacings from the source eliminates some of these adverse response factors.
Depending upon tool calibration conditions, additional corrections must be made. Wireline dual detector thermal neutron porosity tools are typically calibrated in a known formation with a “standard” diameter borehole and with the tool urged against the wall of the borehole. This radial position is commonly referred to as “decentralized” and with no standoff. In logging operations, borehole diameter can vary from “standard”, and the tool can standoff from the borehole wall. Corrections for non standard borehole diameters are typically made in real time using the response of a mechanical wireline caliper. In addition, real time corrections for tool standoff are required to obtain accurate porosity readings. However, standoff measurements are not widely used and ad hoc corrections are typically made based on judgment of borehole conditions. Basic concepts of wireline dual detector thermal neutron porosity logging are disclosed U.S. Pat. No. 4,004,147, which is herein entered into this disclosure by reference. Environmental corrections for wireline dual detector thermal neutron porosity logs are disclosed in the publication “Experimental Determination of Environmental Corrections for a Dual-Spaced Neutron Porosity Log”, D. M. Arnold et al, paper VV, 22nd Annual Logging Symposium Transactions: Society of Professional Well Log Analyst.
Dual detector thermal neutron logging methodology is also applicable to logging-while-drilling (LWD) systems. The basic concepts are the same as those used in the wireline counterpart. LWD tools or subsections are again calibrated in known formations with a “standard” borehole diameter, but with the tool radially centered or “centralized” within the borehole. Unlike the wireline counterpart, mechanical calipers can not be used in LWD systems to measure borehole diameter. Acoustic standoff measurements have been used with fairly good accuracy under most conditions, but can suffer from poor signal if the acoustic waves are not perpendicular to the borehole wall. Acoustic standoff measurements also suffer from inaccuracies due to changes in the mud acoustic properties. Three acoustic sensors placed at 120 degrees from each other are required to obtain a more accurate borehole diameter measurement in LWD systems. Standoff determination from an independent LWD density measurement has been also used with fair accuracy under nominal borehole conditions, but it is adversely affected by changes in the mud density. Moreover, determination of standoff from density measurements is only valid in non-barite mud, which is a major limitation of this approach. Since both the density and the acoustic sensors are focused measurements and “see” only in front of them, borehole diameter measurements in LWD systems generally have less accuracy than standoff measurements. Reliable, real time corrections for borehole diameter and the radial position of the tool within the borehole (i.e. standoff) are needed to obtain accurate LWD neutron porosity measurements.
Measures of epithermal neutrons have been used to enhance or correct dual thermal neutron porosity measurements. Because epithermal neutron flux is typically less than corresponding thermal neutron flux, and because epithermal neutron detectors are less efficient per unit volume than thermal neutron detectors, epithermal detector axial spacing from the neutron source is necessarily smaller to obtain statistically significant measurements. This reduced axial spacing also reduces the radial depth of investigation of the measurement. These factors further discourage the use of dual epithermal neutron detectors at different axial spacings. The use of epithermal neutron measurements to correct thermal neutron porosity measurements has been predominately in the field of pulsed rather than continuous or isotropic neutron sources. Basic concepts of epithermal neutron porosity measurements are disclosed in U.S. Pat. Nos. 5,532,481 (Mickael) and 5,596,191 (Mickael), both of which are herein entered into this disclosure by reference.